Wouldn't mind being in that position myself!. Obviously a lot of personal opinions are going to come into play on this topic, but for the record, here is mine!.
For my requirements, a relatively compact and agile van is essential, so i would be looking at as small a van as i could get away with, and preferably rear wheel drive (better traction, and usually better steering lock). So having had a swb high roof 3.5t iveco daily in the past and loved it (very comfortable, VERY good steering lock, and built like a brick poo house!) that would be my first choice, followed by a transit 330. Obviously if you carry loads of other stuff, and run a 2 or more man setup, you may well need a bigger van though.
For system, given the simplicity of setting one up, i would NEVER even consider buying a proprietary system, they are just not worth it (unless you are trying to impress someone). As long as your chosen tank is fitted securely and your system thought out carefully, there are no drawbacks to a diy setup, but potentially huge financial benefits.
As for heating, although i have not gone down that route myself yet, i would favour the immersion heater element route. The reason for this is i like to keep things simple. 800l-1000l of water heated up to whatever temperature you deem appropriate is, by nature of the volume involved, going to stay warm all day, so why go to the hassle of fitting a gas or diesel water heater, when you don't need to?. From what i have read about immersion elements on this forum, you would have it on for about 6 or 7 hours to heat the water up sufficiently. Simply have it on a timer so the water has heated up for the time you leave to go to work.